High-temperature, high-emissivity, optically black boron surface

ABSTRACT

An article of manufacture comprises a substrate and an optically black surface on the substrate and having an absorptivity of more than about 0.89 and an emissivity more than about 0.86. The surface comprises boron particles plasma spray applied to the substrate using a powder selected from the group consisting of crystalline boron having a particle size finer than about 200 mesh and amorphous boron.

This invention was made with Government support under ContractF33615-81-C-5117 awarded by the Department of the Air Force. Thegovernment has certain rights in this invention.

The present invention relates to optically black boron surfaces whichare applied by plasma-jet spray onto a substrate. For purposes of thepresent application, the term "optically black" means capable of energyabsorption in both the visible (solar) region and the infrared region.In such spectral regions, the surfaces of the present invention haveboth a high absorptivity of solar radiation and a high emissivity in theinfrared region. The surfaces of the present invention also have a highthermal stability, are suitable for use in nuclear environments, and aresubstantially inert to many chemical and oxidizing environments.

BACKGROUND OF THE PRESENT INVENTION

Prior U.S. Pat. No. 4,342,734, issued Aug. 3, 1982 to Kumar and Das,discloses the preparation of thick, dense wafers of crystallinegamma-tetragonal boron by plasma-jet applying beta-rhombohedral boron inpowder form onto a substrate and rapidly cooling the molten particles.The beta-boron powder had a particle size of -100 mesh. The patent makesno mention of the optical properties of the cooled deposit nor referenceto the formation of optically black surfaces. The patent is also silenton other properties such as resistance to high temperature degradationor corrosion resistance to chemical or oxidizing environments.

Prior U.S. Pat. No. 3,231,416, issued Jan. 25, 1966, to Fuller,discloses the preparation of zirconia-boron ablation coatings byplasma-jet spraying. An example of a powder used in the '416 patentcontained 60 volume percent zirconia (325 mesh) and 40 volume percentboron (100 mesh). The coatings were said to be capable of withstandingtemperatures of about 2280° C. during a ten second test period. This, inpart, was attributed to good thermal emittance or ability to radiateheat away from the coated body. As in the '734 patent, no reference ismade to the infrared or visible wavelength absorption or otherproperties which characterize the present invention.

Also of interest are prior U.S. Pat. Nos. 4,503,085, dated Mar. 5, 1985,issued to Dickson et al.; 4,526,618, issued July 2, 1985, to Keshavan etal.; and 4,696,855, dated Sept. 29, 1987, issued to Pettit, Jr., et al.

BRIEF SUMMARY OF THE INVENTION

The present invention resides broadly in an article of manufacturecomprising a substrate and an optically black surface on said substratehaving an absorptivity of more than about 0.89 and an emissivity morethan about 0.86, said surface comprising boron particles plasma sprayapplied to said substrate using a boron powder selected from the groupconsisting of crystalline boron having a particle size finer than about200 mesh and amorphous boron.

Preferably, the crystalline boron has a powder size finer than about 325mesh.

It is also preferable that the boron powder have a purity of at leastabout 95%.

Applications for the articles of manufacture of the present inventioninclude sunshades, surfaces exposed to high temperature environmentssuch as laser beams, thermal control surfaces such as high temperatureradiators for nuclear reactors or neutron absorbers, and surfacesexposed to corrosive chemicals or oxidative agents such as linings forchemical tanks, flow lines and nitrogen tetroxide containing propellanttanks. By the term "sunshade", it is meant any baffle or surfacepositioned for the control of light in such applications as telescopes,laser beam control, energy absorbers or radiators, and spacecraftoptical systems.

Preferred substrates in accordance with the present invention aretitanium, nickel, molybdenum, and alloys thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and advantages thereof will become more apparent uponconsideration of the following specification, with reference to theaccompanying drawings, in which:

FIG. 1 is a surface view of an optically black boron surface prepared byplasma spray application of a crystalline boron powder onto a substrate.The photograph of FIG. 1 was taken at 250X magnification using ascanning electron microscope; and

FIG. 2 is photograph of a portion of the surface of FIG. 1 taken at1,000X magnification using a scanning electron microscope; and

FIG. 3 is a surface view of an optically black boron surface prepared byplasma spray application of amorphous boron onto a substrate. Thephotograph of FIG. 3 was taken at 250X magnification using a scanningelectron microscope.

DETAILED DESCRIPTION OF THE INVENTION AND BEST MODE

The plasma spray process is well known and fully described in numerousprior patents, including, by way of example, prior U.S. Pat. No.4,526,618 referred to above. The disclosure of the '618 patent in thisrespect is incorporated by reference herein.

In the plasma spray process, a gas is used as a heating and carriermedium. A preferred gas is an inert gas such as argon. A stream of thegas is heated to a high temperature by being passed between electricarc-forming electrodes. The plasma expands due to its high heat andkinetic energy and produces a high velocity directional jet. Boronparticles are injected into the rapidly flowing heated stream whereinthey are heated to a sufficiently high temperature to become melted orsoftened. The plasma jet is directed at a target surface, depositing themolten or softened boron particles onto the surface. The boron particlesare allowed to cool by exposure to ambient conditions, aided by theapplication of cooling air to the backside of the substrate. This allowsthe boron particles to become bonded to the surface and to each other inthe formation of a thin layer.

For purposes of the present application, the term "plasma spray process"includes derivatives of the process capable of heating the boronparticles to a high enough temperature to soften or melt the particles,such as detonation gun spraying.

Practically any substrate, including most metals, and even plastics,which are capable of withstanding the high temperature gas stream, andadapted to receive the molten or softened metal particles, can beemployed. Boron has a coefficent of thermal expansion of about 4.6×10⁻⁶in./in./° F., and titanium has a coeffient of expansion very close tothat, about 4.7×10⁻⁶ in./in./° F., making titanium or a titanium alloy apreferred substrate. However, the present invention has beensuccessfully practiced with other substrate surfaces, such as molybdenumand nickel.

It is well known that certain substrates such as molybdenum, tantalum,niobium, tungsten and copper are difficult to coat by plasma spray andobtain a good bond. To further compound the problem, aeronautical andspace uses for the present invention require that the substrates berelatively thin (e.g., less than 0.030 inches) and thus lightweight.This thinness of the substrates makes it impractical to subject them tosevere surface preparation such as abrasion and or substrate heating. Inan aspect of the present invention, it was found that thesedisadvantages could be overcome by applying, by plasma spray, amolybdenum titanium self-bonding coat, preferably a mixture of 80%molybdenum/20% titanium, to which a boron top coat would adhere.

The bond coat could be applied without heating the substrate, adheredwell to such substrates as molybdenum and provided an excellent surfacefor application of a top coat such as boron.

The boron powder employed in the practice of the present invention canbe either a crystalline boron or amorphous boron. Both have beensuccessfully plasma sprayed onto a substrate. Preferably, the boron hasa purity of at least about 90%, more preferably, at least about 95%. Theboron has to be in fine powder form, for instance, that of amorphousboron which typically has an average particle size less than about 10microns. In the case of the crystalline boron, this should have anaverage particle size finer than about 200 mesh, preferably an averageparticle size finer than about 325 mesh.

The fine particle sized boron tended to be hygroscopic and tended topack. This problem was overcome by either forming the boron intoaggregates using an organic binder and mechanically grinding and sievingthe aggregates to a flowable particle size such as -325 mesh, or byadding a small amount of a flow enhancer such as a ceramic oxide to theboron powder to increase its flowability. One suitable binder employedwas polyvinyl alcohol, added as a 2% by weight aqueous solution to thepowder, which was then dried and subjected to grinding and sieving to-325 mesh. The amount of binder used was about five weight percent basedon the weight of the boron. Numerous fine particle size ceramic oxidepowders are commercially available. One that was successfully employedwas a silica ceramic in the form of microspheres marketed by 3M underthe trademark "Glass Bubbles A38/4000". The amount used was about 10-70weight percent, based on the weight of the entire composition. These"Glass Bubbles" had a particle size such that a maximum of 5% by weightwas retained on a U.S. No. 80 standard sieve. Yttria stabilizedzirconium oxide in the amount up to about 70 volume percent (basisentire composition) has also been successfully employed as a ceramicoxide flow enhancing agent. An example of another ceramic oxide isaluminum oxide.

The boron surfaces which are applied to a substrate by the plasma jetprocess have an irregular surface morphology of peaks and valleys whichgives, without further treatment, good absorptivity and emissivitymaking such surfaces useful as optical baffles. However, it was foundthat the absorptivity and emissivity could be further increased bysubjecting the surfaces to calcination. This was carried out at anelevated temperature, for instance about 400° C. to about 600° C. for 30to 60 minutes in an air atmosphere. By calcination, the absorptivity canbe increased up to about 0.95, and the emissivity similarly may beincreased, for instance up to about 0.93.

The coating depths were determined to be about 200±15 microns.

The following examples illustrate the practice of the present invention.In these examples, absorptivity and emissivity were measured using aGier-Dunkle MF-251 solar reflectometer and a Gier-Dunkle DB-100 infraredreflectometer.

EXAMPLE 1

This Example illustrates the preparation of an optically black sunshadeby application of crystalline boron to a substrate. The substrateselected in this Example was a titanium, aluminum, vanadium alloy havingsix parts aluminum and four parts vanadium to 90 parts titanium(Ti-6A1-4V).

The surface of the substrate was prepared by degreasing it with1,1,1-trichloroethane. Application of the boron was carried out using aplasma spray gun manufactured by the Metco Division of Perkin-Elmer,Westbury, N.Y., Model No. 7M.

The boron was crystalline boron marketed by Consolidated Astronauticsidentified as having a purity of 96.6% and a particle size of -325 mesh.Analysis of the powder revealed that it had an average particle size ofabout 20 microns with 90% of the particles being less than about 90microns. To improve the flowability of the boron powder into the plasmaspray gun, the powder was formed into small aggregates by mixing thepowder with 1-2% by weight (based on the weight of boron) of a 2% byweight solution of polyvinyl alcohol, which mixture was then dried. Thedried aggregates were mechanically ground and sieved to -325 mesh (U.S.Sieve).

The spray settings on the plasma spray gun were:

    ______________________________________                                                     Primary Gas  Secondary Gas                                       Parameter    Argon        Hydrogen                                            ______________________________________                                        Pressure     100 psi      50 psi                                              Flow         80 (setting) 15-20 (setting)                                     Nozzle       --           Gh                                                  Current      --           500 Amps                                            Voltage      --           65 V                                                Power        --           32.5 KW                                             Powder Feeder-4MP Dual (Metco)                                                Powder Port          No. 2, 90° orientation                            Powder port shaft    A                                                        Carrier gas flow     37                                                       Air vibrator         15-20 psi                                                Powder feed rate indicator                                                                         80 (setting)                                             ______________________________________                                    

The spraying was carried out using a gun-to-work distance of about 10-15centimeters. A thin film was desired, just covering the substrate, sothat the application was by hand making four fast passes with the gun.The coated substrate was cooled by application of cooling air to thebackside of the substrate. The absorptivity of the boron surface wasabout 0.93 and the emissivity was about 0.87. The coating had a darkgrey appearance and was considered to be suitable, without furthertreatment, for use as an optically black sunshade.

It was found that the optical properties of the surface could besomewhat further increased by exposure of the boron surface tocalcination. In this Example, calcination was carried out at 500° C. to600° C. for about two hours, in air, and resulted in an increase inabsorptivity to about 0.95 (about a 30% decrease in reflectivity), butno increase in emissivity. Following calcination, the coating had ablack appearance.

EXAMPLE 2

This Example illustrates the preparation of an optically black sunshadeusing an amorphous boron. The boron was marketed by ConsolidatedAstronautics and was identified as having a purity of about 96% and anaverage particle size of about five microns. The average particle sizewas determined to be slightly less than five microns.

To increase the flowability of the amorphous boron, the boron was mixedwith 32% by weight, based on the weight of the total composition, offine particle size glass ceramic microspheres marketed by 3M under thetrade designation "Glass Bubbles A38/4000". The microspheres had aparticle size such that a maximum of 5% by weight was retained on a U.S.No. 80 standard sieve.

The spray conditions and equipment used were the same as in Example 1except that the gun to work distance employed was about 5-10centimeters. Also the substrate employed was nickel (Ni-201) instead ofthe titanium alloy of Example 1, and the substrate was prepared, priorto plasma spraying the amorphous boron, by application of a standardnickel bond coat consisting of a nickel, chromium, aluminum alloymarketed by Metco under the trade designation "Metco 443". The bond coatwas plasma spray applied to the nickel substrate using essentially thesame spray conditions as in Example 1, giving a thin layer of about0.002-0.005 inches.

Following spraying, the samples were subjected to calcination under anair atmosphere for about two hours at 600° C. Calcination gave a surfacehaving a visually black appearance and increased the absorptivity fromabout 0.90 to about 0.93. The emissivity following calcination was about0.90. The surface had a visually black appearance.

Samples of optically black baffles prepared in accordance with thisExample were subjected to a continuous wave laser test at 10.6 microns.There was no damage to the boron surface that could be determined byvisual inspection. Absorptivity decreased slightly from about 0.93 toabout 0.92. Emissivity decreased slightly from about 0.90 to about 0.89.

Example 3

Similar samples as in Example 2 were prepared using. molybdenum as thesubstrate.

It is well known that materials such as molybdenum, tantalum, niobium,tungsten and copper are difficult materials on which to depositself-bonding surface coatings by plasma spray. In the present instance,the uses of principle interest are space or aeronautical associated,where light weight is desired. Accordingly the substrate in this examplehad a thickness of only about 0.015 inches. This prevented the surfacefrom being intensively prepared, for instance by surface roughening, orheated to an elevated temperature prior to applying a plasma sprayedboron coat.

It was found that these difficulties could be overcome by applying amolybdenum/titanium bond coat to the substrate prior to application ofthe boron top coat. Only through cleaning of the substrate was required,and no mechanical roughening of the surface such as by rough gritblasting was necessary. The bond coat adhered well and provided asurface roughness ideally suited for the reception of sprayed top coats.

In this example, the bond coat was an 80/20 blend of molybdenum andtitanium. The substrate was cleaned using 1,1,1-trichloroethane and alight sand blast. The conditions of application and spray gun were:

    ______________________________________                                                  Spray Gun                                                                     Type - 7M (Metco)                                                             Nozzle - GH                                                                   Uni-jet Ring                                                                  Argon Insulator                                                               #2 Powder Port at 90°                                        Gas Pressure     Gas Flow                                                     Primary (argon) - 100 psi                                                                      Primary (argon) - 80 psi                                     Secondary (hydrogen) - 50 psi                                                                  Secondary (hydrogen) - 15 psi                                Power            Powder Feed                                                  Unit Model - 7MR (Metco)                                                                       Unit - 4MP Dual (Metco)                                      Arc Amps - 500   Powder Port Shaft - A                                        Arc Volts - 60-65                                                                              Carrier Gas Pressure - 100 psi                                                Flow Meter Reading - 47 psi                                                   Feed Rate Indicator - 100-125                                                 Air Vibrator Pressure - 20-25                                                 PSI                                                          ______________________________________                                    

The thickness of the bond coat was about 0.002-0.005 inches.

It should be noted that this bond coat can be successfully applied toalmost any metal, and was successfully applied to such substrates asnickel, titanium and the titanium-aluminum vanadium alloy of Example 1,in addition to molybdenum, giving in each instance excellent bondstrength and the advantages noted above with regard to application to amolybdenum substrate.

In this Example, the same spray conditions and amorphous boron asemployed in Example 2 were used. The amorphous boron surface when plasmasprayed onto the bond coat gave an absorptivity of about 0.97 andemissivity of about 0.88. These samples also were deemed to be withinthe scope of the present invention. No posttreatment such as calcinationwas used.

These samples were exposed to the same continuous wave laser test as inExample 2, and the samples also passed visual examination. Theabsorptivity was found to decrease slightly to about 0.93, whereas theemissivity remained generally constant.

The sample of Example 3 was the subject of the photograph of FIG. 3,taken at 100×magnification. As shown in this Figure, the boron particlesformed, on impact, a rough, nodular, layered surface with deepindentations capable of trapping light.

EXAMPLE 4

In this Example, the same crystalline boron as in Example 1 was plasmaspray applied to a nickel (Ni-201) substrate (precoated with a nickel,chromium, aluminum alloy bond coat, Metco 443, as in Example 2),subsequently calcined, and then exposed to the same continuous wavelaser test as in Examples 2 and 3. To increase the flowability of thecrystalline boron, the boron was mixed with 32% by weight of glassceramic microspheres as in Example 2. The procedures of plasma sprayapplication and calcination were the same as in Example 1. Followingplasma spray application and following calcination, the solarabsorptivity was determined to be about 0.92 and about 0.97,respectively. The emissivity after plasma spray application and aftercalcination remained constant at about 0.88. Following the continuouswave laser test, absorptivity dropped slightly to about 0.93 andemissivity decreased slightly to about 0.87. These samples were deemedalso to be within the scope of the present invention, and were deemed tohave passed the laser test, both with regard to measured absorptivityand emissivity, and by visual examination.

The physical characteristics of the surfaces of this Example are shownin FIGS. 1 and 2, taken at 250×and 1,000×magnification, respectively. Asshown in these Figures, the boron particles formed on impact, a rough,platelet layered surface with deep indentations capable of trappinglight.

In the surfaces of both Examples 3 and 4, the nodules or plateletsprovided a large number of points of absorption separated from eachother by a spectrum of distances roughly equivalent to or slightly morethan the spectrum of visual and infrared light. Although not to be heldto any particular theory, it is believed that the high absorptivity andemissivity values obtained in the practice of the present invention aredue to this, plus the blackness of the relatively pure boron particles.

EXAMPLES 5-10

These Examples illustrate that the surfaces of the present inventionhave a resistance to attack from common corrosive or oxidative agentssuch as acids and solvents.

The samples were prepared following the procedure of Example 1 exceptthat there was no calcination posttreatment following plasma sprayapplication to a substrate. The substrate in these Examples were thetitanium, aluminum, vanadium alloy of Example 1. The boron form employedwas the same crystalline boron as in Example 1.

Exposure to the corrosive agents was carried out employing conventionaletching procedures; namely, maintaining the agents at room temperatureor an elevated temperature as indicated in the following table, and thenholding the samples immersed in the solutions for a sufficient period oftime to cause etching if etching was to occur, for instance as evidencedby the evolution of hydrogen. In all instances, there was no evidence ofetching.

The following Table gives the results obtained:

                  TABLE 1                                                         ______________________________________                                                                            Effect on                                                       Optical Properties                                                                          Optical                                                         Ab-      E-     Proper-                                 Samples Treatment     sorptivity                                                                             missivity                                                                            ties                                    ______________________________________                                        Example 5                                                                             Exposure to a --       --     None                                            methanol/                                                                     sulfuric acid                                                                 mixture                                                               Example 6                                                                             Exposure to an                                                                              0.90     0.87   None                                            ethanol/                                                                      potassium hydrox-                                                             ide mixture                                                           Example 7                                                                             Exposure to a sul-                                                                          --       --     None                                            furic acid/chromic                                                            oxide solution                                                        Example 8                                                                             Exposure to boiling                                                                         0.91     0.89   None                                            sulfuric acid                                                         Example 9                                                                             Exposure to boiling           None                                            nitric acid                                                           Example 10                                                                            Exposure to nitric                                                                          0.89     0.86   None                                            acid/sodium chro-                                                             mate solution                                                         ______________________________________                                    

Similar samples subjected to conventional anodization also showed aresistance or inertness to chemical reactivity under these conditions.

The low chemical reactivity characteristics of the surfaces of thepresent invention permits them to be exploited in many corrosivechemical container and flow line applications, as well as for equipmentwhich must be operated in a corrosive or oxidative environment.

EXAMPLE 11

The purpose of this Example is to show that the boron surfaces of thepresent invention are stable to prolonged exposure at high temperatures.In this Example, crystalline boron was plasma spray applied to thetitanium alloy substrate of Example 1, except that the substrate wasfirst bond coated with a nickel, chromium, aluminum alloy (Metco 443) asin Example 2. The boron powder was blended with 32% by weight glassceramic microspheres as in Example 2 to increase flowability. The sameconditions of application, procedures, boron powder, and spray gun as inExample 1 were used. The boron surface, following application, had anabsorptivity of about 0.91 and an emissivity of about 0.88.

The surface was subjected to calcination at 900° C. for 1/2 hour.Absorptivity actually increased slightly to about 0.92, and emissivityremained constant at about 0.88. The surface remained visually black.

Having described a preferred embodiment of the invention, I claim:
 1. Anarticle of manufacture which comprises a substrate and a boron coatingadhered to said substrate, said boron coating being optically black andhaving an absorptivity of more than about 0.89 and an emissivity morethan about 0.86, said boron coating being obtained by plasma sprayingonto said substrate a boron powder selected from the group consisting ofcrystalline boron having a particle size of about -200 mesh andamorphous boron under conditions effective to obtain adhesion of saidcoating to said substrate and to obtain said absorptivity andemissivity.
 2. The article of claim 1 wherein said crystalline boron hasa particle size of about -325 mesh.
 3. The article of claim 1 whereinsaid boron powder has a purity of at least about 90%.
 4. The article ofclaim 1 wherein said substrate is selected from the group consisting oftitanium, nickel, molybdenum and alloys thereof.
 5. A sunshadecomprising the article of manufacture of claims 1, 2, 3 or
 4. 6. Athermal control surface comprising the article of manufacture of claims1, 2, 3 or
 4. 7. A lining for chemical tanks or flow lines comprisingthe article of manufacture of claims 1, 2, 3 or
 4. 8. An article ofmanufacture of claim 1 having a boron surface thereon substantially asshown in the photograph of FIG.
 1. 9. An article of manufacture of claim1 having a boron surface substantially as shown in the photograph ofFIG.
 2. 10. An article of manufacture of claim 1 having a boron surfacesubstantially as shown in the photograph of FIG.
 3. 11. The article ofclaim 1 which is calcined subsequent to plasma spraying.
 12. Anoptically black sunshade comprising a substrate and a boron coatingadhered to said substrate, said boron coating being optically black andhaving a solar absorptivity of more than about 0.89 and an emissivity ofmore than about 0.86, said boron coating being obtained by plasmaspraying onto said substrate a boron powder selected from the groupconsisting of crystalline boron having a particle size of about -200mesh and amorphous boron under conditions effective to obtain adhesionof said coating to said substrate and to obtain said absorptivity andsaid emissivity.
 13. The sunshade of claim 12 wherein said substrate isa compound selected from the group consisting of molybdenum, titanium,nickel, and alloys thereof.
 14. The sunshade of claim 13 wherein saidboron is crystalline boron having a purity of at least about 90%. 15.The sunshade of claim 13 wherein said boron is amorphous boron having apurity of at least about 90%.
 16. The sunshade of claim 14 or 15 whereinsaid boron is in the form of fine particles blended with ceramic oxidemicrospheres.
 17. An article of manufacture which comprises a substrateand a boron coating adhered to said substrate, said boron coating beingoptically black and having an absorptivity of more than about 0.89 andan emissivity more than about 0.86, said boron coating being obtained byplasma spraying onto said substrate a boron powder selected from thegroup consisting of crystalline boron having a particle size of about-200 mesh and amorphous boron under conditions effective to obtainadhesion of said coating to said substrate and to obtain saidabsorptivity and emissivity, said substrate having a coefficient ofthermal expansion close to that of boron.
 18. The article of claim 17wherein said substrate is titanium or a titanium alloy.
 19. An articleof manufacture which comprises a substrate, a bond coat adhered to saidsubstrate, and a boron coating adhered to said bond coat, said boroncoating being optically black and having an absorptivity of more thanabout 0.89 and an emissivity more than about 0.86, said boron coatingbeing obtained by plasma spraying onto said bond coat a boron powderselected from the group consisting of crystalline boron having aparticle size of about -200 mesh and amorphous boron under conditionseffective to obtain adhesion of said coating to said substrate and toobtain said absorptivity and emissivity.
 20. The article of claim 19wherein said bond coat comprises molybdenum and titanium.
 21. Thearticle of claim 20 wherein said bond coat comprises a mixture of 80%molybdenum and 20% titanium.
 22. The article of claim 21 wherein saidsubstrate has a thickness less than about 0.030 inch.